Frequency control method in a CDMA communication system for preventing deterioration in communication quality despite failure to control of transmission power in a mobile station

ABSTRACT

A frequency control method in a CDMA communication system is described that prevents deterioration in communication quality even in cases in which a mobile station loses control of transmission power. A mobile station is determined to have failed when the difference between the reception Eb/I 0  and a reference Eb/I 0  exceeds a large predetermined value, whereupon the base station instructs the mobile station to turn off its power supply. If the power supply of that mobile station cannot be confirmed to be off, the base station instructs the mobile station to change its frequency. If the frequency change of that mobile station cannot be confirmed, the base station instructs normal mobile stations other than the failed mobile station to change frequency.

This is a continuation of application No. 09/356,680, filed Jul. 19,1999; U.S. Pat. No. 6,487,192; the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a CDMA (Code Division Multiple Access)Communication System, and more particularly to a frequency controlmethod for controlling the frequency of channels established between thebase station ad a mobile station.

2. Description of the Related Art

In recent years, the CDMA communication method, which is effective incontrolling interference and disturbance, is receiving increasingattention as a communication method used in mobile communicationsystems. In such a CDMA communication method, a user signal that is tobe transmitted is spread by a spreading code on the transmission side tobe transmitted, and the original user signal is obtained on thereceiving side by using a spreading code identical to the spreading codefor despreading.

In the CDMA communication system, a plurality of transmission sideseffect spreading by using different spreading codes each havingorthogonality, and at the receiving sides, each communication can beidentified by selecting the spreading code which is used at despreading,thereby enabling a plurality of communications sharing the samefrequency band.

However, since it is difficult to maintain absolute orthogonalitybetween all of the spreading codes that are used, each of the spreadingcodes are actually not perfectly orthogonal and have correlationcomponents with other codes. These correlation components becomeinterference components in that communication and bring aboutdeterioration in communication quality. This produces an interferencecomponent which increases with an increase in the number ofcommunications.

In general, a certain signal-to-noise ratio is needed in a radiocommunication system to secure communication quality. For example, in aspread spectrum communication system, a certainsignal-to-(noise+interference) ratio is needed to secure communicationquality. This value is usually referred to as Eb/I0, where Eb is adesired reception wave power and I0 is an interference wave power. TheEb/I0 needed to secure the certain communication quality is alsoreferred to as a required Eb/I0.

As described hereinabove, however, for one particular receiver,transmission signals sent to another receiver are noise components andgenerate interference to the desired signals transmitted to thereceiver. As a result, Eb/I0 can be secured most efficiently when thepowers of a plurality of transmission signals received at a receiver areall the same. For this reason, in a CDMA communication system, the basestation controls the transmission power of each mobile station such thatthe Eb/I0 obtained from the communications of each of the mobilestations reaches a reference Eb/I0. The reference Eb/I0 is a value thatis set based on a required Eb/I0, and is typically substantially equalto the reference Eb/I0.

Specifically, the base station instructs each mobile station to increaseor decrease the present transmission power based on transmission powercontrol signals for instructing the increase or decrease of transmissionpower to mobile stations contained in the downward channel transmittedfrom the base station to mobile stations.

However, when a failure occurs at a mobile station, the control oftransmission power may be impossible, and in some cases transmissionpower continues to rise regardless of the transmission power control bythe base station.

When this happens, the reception Eb/I0 of mobile stations other than themobile station in which the failure occurred is degraded by theinfluence of the communication of the mobile station in which failureoccurred, so that the reference Eb/I0 can no longer be secured.Therefore, the base station instructs each mobile station in whichfailure has not occurred to increase transmission power of the mobilestation, and each mobile station performs communications at atransmission power greater than needed at each mobile station, thusshortening the battery life. In a worst case, all mobile stations withina cell communicate at the greatest transmission power that can betransmitted. As a result, the required Eb/I0 cannot be obtained at allmobile stations within the cell, making all mobile stations within thecell lose the ability to communicate.

In other words, the CDMA communication system of the prior art hassuffered from problem that, should control of transmission power be lostat any particular mobile station and transmission power rise to anunnecessarily high level, transmission quality at other mobile stationswithin the cell can no longer be maintained.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a frequency controlmethod in a CDMA communication system in which, even when a particularmobile station loses control of transmission power and hence anundesired interference wave is produced within the cell, thisinterference wave does not cause deterioration in the communicationquality of other mobile stations in which failure has not occurred.

To achieve the aforementioned object, in the frequency control method ina CDMA system according to the invention, a CDMA base station determinesthat a particular mobile station has failed to control of transmissionpower if the difference between a reference Eb/I0 and the receptionEb/I0 of the communication channel that is established between the basestation and the mobile stations has exceeded a predetermined value, andinstructs the mobile station to turn off its power supply.

In the present invention, a CDMA base station instructs a particularmobile station to turn off its power supply in the event of a failure atthat mobile station that loses control of transmission power, causingthe transmission power of that mobile station to rise above thenecessary level. The invention thus prevents deterioration in thecommunication quality of other normal mobile stations in which failureshave not occurred due to the influence of transmission signals from theproblem mobile station. In addition, the invention provides a solutionto the problem of shortened battery life when a particular mobilestation loses control of transmission power and other normal mobilestations communicate at transmission power greater than the necessarytransmission power.

In another frequency control method in a CDMA system of the invention, aCDMA base station determines that a particular mobile station has failedto control of transmission power if the difference between a referenceEb/I0 and the reception Eb/I0 of the communication channel establishedbetween the base station and the mobile stations has exceeded apredetermined value, and instructs that particular mobile station tochange the frequency used as the communication channel to anotherfrequency.

In this invention, a CDMA base station instructs a particular mobilestation to change the frequency used as the communication channel toanother frequency when a failure has occurred at that particular mobilestation, so that mobile station loses control of transmission power, andthe transmission power of that mobile station rises above the necessarytransmission power. The invention thus prevents deterioration in thecommunication quality of other normal mobile stations in which failureshave not occurred caused by the influence of transmission signals fromthat mobile station.

In addition, according to another frequency control method in a CDMAsystem of the present invention, a CDMA base station determines that aparticular mobile station has failed to control of transmission power ifthe difference between a reference Eb/I0 and the reception Eb/I0 of thecommunication channel established between the base station and themobile station has exceeded a predetermined value, and instructs normalmobile stations other than that particular mobile station to change thefrequency used as the communication channel to another frequency andalso change the frequency used as the communication channel to theanother frequency.

In this invention, when a failure occurs at a particular mobile station,so that the particular mobile station loses control of transmissionpower, and the transmission power of that mobile station rises above thenecessary transmission power, a CDMA base station instructs normalmobile stations within the cell other than that mobile station to changethe frequency used as the communication channel to another frequency andalso change the frequency used as the communication channel to theanother frequency. Accordingly, even when instructions from the basestation can not be received by the mobile station that has failed tocontrol of transmission power, deterioration in communication qualitydue to the influence of transmission signals from that mobile stationcan be prevented for other normal mobile stations in which no problemsare occurred.

According to another frequency control method in a CDMA system of thepresent invention, a combination of any two or all three of theforegoing frequency control methods may be used.

According to the present invention, if the power supply of a mobilestation in which a failure has occurred does not turn off despiteinstructions from the base station, the frequency used as thecommunication channel by that mobile station is changed; and if thefrequency change is impossible, the frequency employed as thecommunication channel by other normal mobile stations and the CDMA basestation is changed to another frequency. Thus, by using a methoddepending on the severity of the problem that has occurred in the mobilestation that has failed to control of transmission power, the inventioncan prevent deterioration in communication quality in other normalmobile stations in which problems have not occurred caused by theinfluence of transmission signals from that mobile station.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings which illustrate examples of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the construction of a typical CDMAcommunication system;

FIG. 2 is a flow chart showing the operation of a CDMA communicationsystem according to a first embodiment of the present invention;

FIG. 3 is a flow chart showing the operation of a CDMA communicationsystem according to a second embodiment of the present invention;

FIG. 4 is a flow chart showing the operation of a CDMA communicationsystem according to a third embodiment of the present invention; and

FIG. 5 is a flow chart showing the operation of a CDMA communicationsystem according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a typical CDMA communication system comprises abase station 1 and a plurality of mobile stations 2 ₁-2 _(n). Mobilestations 2 ₁-2 _(n) each communicate with base station 1 based on a CDMAsystem by establishing a communication channel of the same frequencybetween mobile stations 2 ₁-2 _(n) and base station 1. Base station 1and mobile stations 2 ₁-2 _(n) are of a generally known construction,and detailed description of their compositions is therefore hereinomitted.

First Embodiment

The operation of the CDMA communication system of this embodiment willbe described with reference to the flow chart shown in FIG. 2, by way ofexample in which a failure has occurred in mobile station 2 ₁ within aservice area, so that control of transmission power is lost, and thetransmission power can no longer be decreased.

When signals transmitted from mobile stations 2 ₁-2 _(n) are received atbase station 1 at step 101, base station 1 compares reception Eb/I0 ofsignals transmitted from mobile stations 2 ₁-2 _(n) with the value of areference Eb/I0 previously set by the system, and includes atransmission power control (TPC) signal to lower the transmission powerin transmission signals from base station 1 if the reception Eb/I0 ishigher than the reference Eb/I0.

In step 102, when signals transmitted from base station 1 are receivedat mobile stations 2 ₁-2 _(n), mobile stations 2 ₁-2 _(n) analyze thetransmission power control signals transmitted from base station 1, andtransmits a signal at decreased transmission power if instructed todecrease transmission power.

Thus, increasing and decreasing the transmission power of mobilestations 2 ₁-2 _(n) makes it possible to control the reception Eb/I0 inthe communication channel between base station 1 and each of mobilestations 2 ₁-2 _(n) converge with the reference Eb/I0 set in advance bythe system.

In step 103, base station 1 makes a check to determine if the differencebetween the reference Eb/I0 and the reception Eb/I0 of mobile stations 2₁-2 _(n) at base station 1 is greater than a particular value “a”. If itis found the difference between the reference Eb/I0 and the receptionEb/I0 at base station 1 of a particular mobile station 2 ₁ exceeds aparticular value “a” in step 103, then base station 1 determines thatmobile station 2 ₁ has failed to control of transmission power. In thiscase, value “a” is a relatively large value, i.e., a value that wouldnot be obtained if transmission power control is being carried outnormally. In step 104, base station 1 then transmits an instruction tomobile station 2 ₁, which has failed to control of transmission power,to turn off its power supply.

The power supply of mobile station 2 ₁ is then turned off in accordancewith the “power supply off” instruction from base station 1, therebyallowing prevention of deterioration of communication quality of theother normal mobile stations 2 ₂-2 _(n) caused by transmission signalsfrom mobile station 2 ₁. In addition, the problem that battery life isshared by the fact that other normal mobile stations 2 ₂-2 _(n)communicate at transmission power that is greater than necessary, canthus be solved even when control of transmission power becomesimpossible at mobile station 2 ₁.

Second Embodiment

A frequency control method in a CDMA communication system according tothe second embodiment of the present invention is next explained withreference to FIG. 3.

The present embodiment is a modification of the first embodiment shownin FIG. 2 in which step 105 has been substituted for step 104.

In this embodiment, if the difference between the reception Eb/I0 andreference Eb/I0 exceeds value “a” in step 103 and a particular mobilestation is determined having a failure, base station 1 instructs mobilestation 2 ₁, which has failed to control of transmission power, tochange the frequency used as the communication channel in step 105. Thisprevents the deterioration in communication quality for other normalmobile stations 2 ₂-2 _(n) caused by transmission signals from mobilestation 2 ₁, in which transmission power control has been lost. Thisembodiment also solves the problem that battery life is shortened by thefact other normal mobile stations 2 ₂-2 _(n) communicate at greatertransmission power than is necessary, even when control of transmissionpower has become impossible in mobile station 2 ₁.

Third Embodiment

A frequency control method in a CDMA communication system according tothe third embodiment of the present invention will be described withreference to FIG. 4.

In the first and second embodiments, the output of an instruction fromthe base station to a mobile station that has failed to control oftransmission power to turn off the power supply or to change frequencyprevents deterioration in the communication quality of other normalmobile stations caused by transmission signals from the mobile stationin which failure has occurred. Since mobile station 2 ₁, in whichcontrol of transmission power has failed, is experiencing some form offailure, however, the output of an instruction from base station 1 doesnot necessarily mean that mobile station 2 ₁ is controlled according tothe instruction.

This embodiment enables prevention of deterioration of communicationquality of other normal mobile stations 2 ₂-2 _(n) caused bytransmission signals from mobile station 2 ₁ even when mobile station 2₁ loses control of transmission power.

The flow chart of FIG. 4 is a modification of FIG. 2 in which steps 106and 107 have been replaced for step 104.

Steps 101-103 are the same as those in FIG. 2, and explanations thereofwill therefore be omitted.

In this embodiment, if the difference between reception Eb/I0 andreference Eb/I0 exceeds value “a” and a particular mobile station isdetermined to be experiencing a failure in step 103, base station 1instructs normal mobile stations 2 ₂-2 _(n) other than mobile station 2₁, which has failed to control of transmission power, to change thefrequency used as the communication channel in step 106. Then, in step107, base station 1 and normal mobile stations 2 ₂-2 _(n) switch each offrequencies used as the communication channel to another frequency.

This embodiment can thus prevent deterioration in the communicationquality of other normal mobile stations 2 ₂-2 _(n) caused bytransmission signals from mobile station 2 ₁, which has failed tocontrol of transmission power, even when mobile station 2 ₁ fails toaccept control signals from base station 1. This embodiment also solvesthe problem that battery life is shortened by the fact other normalmobile stations 2 ₂-2 _(n) communicate at greater transmission powerthan is necessary, even when control of transmission power has becomeimpossible in mobile station 2 ₁.

Fourth Embodiment

A frequency control method in a CDMA communication system according tothe fourth embodiment of the present invention will be next explainedwith reference to FIG. 5, which is a flow chart illustrating theoperation of the CDMA communication system.

The frequency control method of this embodiment is a combination of thefrequency control methods in the above-mentioned first to thirdembodiments.

The frequency control method of this embodiment is identical to thefrequency control method of the first to third embodiments as far assteps 101-103, and explanations thereof will therefore be omitted.

In this embodiment, if the difference between the reception Eb/I0 andreference Eb/I0 exceeds value “a” and a particular mobile station isdetermined to have failed in step 103, base station 1 sends aninstruction in step 104 to mobile station 2 ₁ that is determined to havelost control of transmission power to turn the power supply off.

Base station 1 then determines in step 108 whether or not the powersupply of mobile station 2 ₁ is confirmed to be turned of. If it isconfirmed that the power supply of mobile station 2 ₁ is turned off instep 108, base station 1 executes the processing of steps 101 and 102,in which transmission power is controlled as normal; but if base station1 cannot confirm that the power supply of mobile station 2 ₁ is turnedoff in step 108, base station 1 executes the processing of step 105. Instep 105, base station 1 instructs mobile station 2 ₁, which has failedto control of transmission power, to change the frequency used as thecommunication channel.

If the frequency change of mobile station 2 ₁ can be confirmed in step109, base station 1 then executes the processing of steps 101 and 102,in which normal transmission power control is carried out; but if thefrequency change of mobile station 2 ₁ cannot be confirmed in step 109,base station 1 executes step 106.

In step 106, base station 1 instructs normal mobile stations 2 ₂-2 _(n)other than mobile station 2 ₁, which has failed to control oftransmission power, to change the frequency employed as thecommunication channel. In step 107, base station 1 and normal mobilestations 2 ₂-2 _(n) then each switch the frequency employed as thecommunication channel to another frequency.

In this embodiment, if the power supply of mobile station 2 ₁, in whicha failure has occurred dose not turned off despite instructions frombase station 1, the frequency used as the communication channel by thatmobile station 2 ₁ is changed; and if this change cannot be effected,the other normal mobile stations 2 ₂-2 _(n) and base station 1 thenchange the frequency used as the communication channel to anotherfrequency. Accordingly, the influence of transmission signals frommobile station 2 ₁ can be prevented from causing deterioration in thecommunication quality of other normal mobile stations 2 ₂-2 _(n) inwhich no problems have occurred by using a method according to theseverity of the failure that has occurred in mobile station 2 ₁, whichhas failed to control of transmission power. In addition, even whenmobile station 2 ₁ loses control of transmission power, the inventioncan solve the problem battery life shortened due to the fact that othernormally operating mobile station 2 ₂-2 _(n) communicate at greatertransmission power than is necessary.

Although this embodiment uses a combination of all three frequencycontrol methods in the above-described first to third embodiments, acombination of any two of the frequency control methods may also beused. For example, the method in which mobile station 2 ₁, which hasfailed to control of transmission power, is instructed by base station 1to turn off its power supply may be combined with the method in whichthe frequency of normal mobile stations 2 ₂-2 _(n) is changed. In thiscase, the process of step 106 may be executed after the process of step108 in the flow chart of FIG. 5.

In the aforementioned first to fourth embodiments, explanation has beenpresented regarding a case in which a failure occurs and control oftransmission power is lost in only one mobile station 2 ₁, but thepresent invention is not limited to such a case and may be similarlyapplied to cases in which failures occur and control of transmissionpower is lost in a plurality of mobile stations.

While preferred embodiments of the present invention have been describedusing specific terms, such description is for illustrative purposesonly, and it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

1. A frequency control method in a CDMA communication system forcontrolling the frequency of a communication channel to be establishedbetween a CDMA base station and a plurality of mobile stations thatcommunicate with said CDMA base station, comprising: determining that aparticular mobile station among said plurality of mobile stations hasfailed to control its transmission power when the difference between areference Eb/I0 and reception Eb/I0 of a communication channelestablished between said particular mobile station and said CDMA basestation exceeds a predetermined value; and instructing said particularmobile station to turn off its power supply, wherein Eb is a desiredreception wave power and I0 is an interference wave power.
 2. Afrequency control method in a CDMA communication system for controllingfrequency of a communication channel to be established between a CDMAbase station and a plurality of mobile stations that communicate withsaid CDMA base station, comprising: determining that a particular mobilestation among said plurality of mobile stations has failed to controlits transmission power when the difference between a reference Eb/I0 andreception Eb/I0 of a communication channel established between saidparticular mobile station and said CDMA base station exceeds apredetermined value; and instructing said particular mobile station tochange the frequency employed as communication channel to anotherfrequency, wherein Eb is a desired reception wave power and I0 is aninterference wave power.